The present invention relates to a device for cutting labels in a labelling machine of the type described in the preamble of present claim 1.
As known, the cutting roller finds its most extensive use to cut into segments or labels a thin film of polypropylene, PVC or plastic film in general, whereon are printed the images and data constituting the label.
It could nonetheless also be used for films made of other materials, for instance paper. Labelling machines that employ label films generally comprise:                an assembly for unwinding the reeled film;        a cutting assembly constituted by a vertical blade mounted on a roller rotating about a vertical axis, which in the technical jargon is called cutting roller, and by a fixed vertical counter-blade;        a drum for picking up and transferring the cut label towards a roller for spreading the glue which intervenes in predetermined areas of the label.        
Both the cutting roller, bearing the blade, and the pick-up drum are provided with internal chambers in which a vacuum is created and which communicate, through a plurality of holes, with the outer surface of the cutting roller or of the drum. Such a vacuum attracts and holds the labels while they are cut and then transferred onto the drum; the vacuum also retains the labels while they are on the drum itself.
The cutting of the film, which in some cases has a thickness of a few hundredths of a millimetre, presents several problems due mainly to the flexion or deformation of the support of the cutting roller.
Said flexion or deformation occurs mainly because of the unequal and inconstant heat expansions in the whole cutting roller. In particular it is evident that, at the start of the work process, the supports of the cutting roller have low temperatures which progressively increase as the hours of work increase, so that an operator is forced to adjust the position of the counter-blade during the working period.
Said flexion or deformation is caused by the frequent impact of the blade mounted on the roller against the counter-blade, the impact being necessary for cutting the labels.
According to a first prior art, to overcome the aforesaid drawback, the same Applicant has provided a cutting roller which comprises ducts able forcedly to carry oil, heated at a temperature exceeding that of the external environment by about 20-30° C., into the roller support frame. This allows to heat said support frame to a temperature of about 50° C. before starting the labelling cycle, and to maintain said temperature during the work of the machine.
A cutting roller heated in accordance with said first prior art has the important drawback of increasing the operating costs of the labelling machine. To heat the oil, it is necessary to provide an adequate heating system connected to the ducts obtained in the roller support frame.
Disadvantageously, an external heating system considerably increases the size of the labelling machine and requires particular maintenance.
According to a second prior art, from document EP 1177981 of the same Applicant it is known a cutting roller comprising a cooling line for the two support systems of the shaft of the cutting roller. The cooling line is preferably independent for each of the two systems and it comprises a probe for measuring the temperature of the related support which sends its measured value to a block for its comparison with a pre-set preferential temperature value. The block, according to the compared value, activates a solenoid valve inserted on a compressed air supply circuit connected to cooling line of the cutting roller.
More precisely, the cutting roller comprises, in correspondence with the two supports of its rotation shaft, ducts wherein compressed air at ambient temperature is sent. In correspondence with said supports are preferably provided temperature detection probes which activate or inhibit the flow of air by acting on appropriate solenoid valves inserted on the compressed air supply line.
The main drawback of a cutting roller obtained in accordance with said second prior art is that the use of compressed air does not allow an efficient heat exchange. Essentially, compressed air at ambient temperature is not able to absorb sufficient heat from the roller and from the frame, making said cooling system poorly efficient at high rates of rotation of the roller.
In accordance with a third prior art, there are cutting rollers cooled by oil circulating inside ducts obtained in the roller support frame.
In particular, an oil-cooled cutting roller requires an external pump able to send pressurised oil inside the ducts. Specifically, the pump is usually pneumatic and it comprises a piston, whose alternating motions necessary for sending the oil is actuated by alternating phases of intake and expansion of a gaseous fluid, typically air.
The main drawback of such a cutting roller is that the volumetric flow rate of oil delivered by the external pump is independent of the rotation rate of the roller. In particular, the volumetric flow rate of oil delivered into the ducts remains constant at any rotation rate of the cutting roller and, once it is set, it can no longer be varied. Essentially, the pump operates at constant speed, regardless of the angular velocity of the roller.
Consequently, this technical solution is not able to assure a constant and efficient cooling of the cutting roller, because, as the rotation rate of the roller changes, it volumetric flow rate of the cooling oil cannot be changed.
An additional drawback of said third prior art is represented by the need to use a control system able to identify any malfunctions of the pump, which makes the labelling machine costlier and more complex.